The present invention generally relates to fiber optic hydrophone sensors used in seismic offshore mineral exploration and, more particularly, is concerned with a fiber-optic hydrophone sensor with improved performance and life when used in severe environments of high hydrostatic pressures.
The concept of using an optical fiber in sensing applications is not new. The U.S. Naval Research Laboratory (NRL) has been a leader in this area. NRL and others have disclosed a number of optical systems. U.S. Pat. No. 4,648,083 to Tom Gialorenzi of the Naval Research Lab, incorporated herein by reference, describes a typical fiber optic system. In this case optical phase equivalent to acoustic pressure in a hydrophone was measured. Common fiber optic hydrophone sensors consist of coils of optical fiber wrapped around mandrels. U.S. Pat. No. 4,525,818 to Cielo et al., incorporated herein by reference, illustrates such a fiber optic hydrophone. The fiber optic coils are attached to optical couplers to create an interferometer. The physical phenomenon being measured is directly converted into differential optical phase by acting on the interferometer. The acoustic pressures act on the arms of the interferometer creating an optical phase shift in the interferometer. U.S. Pat. No. 5,363,342 to Layton et al., incorporated herein by reference, and U.S. Pat. No. 5,285,424 to Meyer, incorporated herein by reference, discusses the fiber optic hydrophone in more detail. In this case the two arms of the interferometer are wound around two separate mandrels, one placed inside the other, creating a concentric mandrel configuration. An air cavity between the two mandrels is used to enhance the sensitivity of the hydrophone.
Another optical approach consists of fiber Bragg grating based sensors. The fiber Bragg gratings can be used in different manners to measure a given phenomenon. The first method is to use the grating as reflector, creating a Fabry-Perot or Michelson interferometer. With the Fabry-Perot interferometer a similar change in the phase of the light is measured. In the second method the grating itself is the sensor. Strain on the grating changes the period of the grating, which changes the wavelength of light reflected from the grating. This wavelength change is proportional to the strain on the grating.
FIG. 1 shows a typical hydrophone sensor. The sensor, generally designated G, is a pressure sensor and is typically used to measure acoustic pressures in water-covered areas. The sensor G consists of an outer, sensing mandrel A that is compliant and is wrapped with an optical fiber B around its outer circumference. The sensor G also has an inner, reference mandrel C that is rigid and wrapped with an optical fiber D around its outer circumference. The mandrels are attached to end caps E on each end with epoxy or urethane sealant to prevent air in the air cavity F between the compliant and rigid mandrel from escaping. The sensor G is placed in the vicinity of an acoustic seismic source. The acoustic source generates an acoustic wave in the water. The reflected acoustic wave acts on the sensor G. The wave""s pressure variation produces a temporary deformation of the compliant sensing mandrel A, as illustrated by the dashed line in FIG. 1. The optical fiber coil B wrapped around the sensing mandrel stretches and contracts in relation to changes in shape of the sensing mandrel A. Light traversing the optical fiber B on the sensing mandrel A travels a slightly longer distance when the fiber is stretched due to deformation of the sensing mandrel A. However, the reference mandrel C is rigid, and is also acoustically isolated from the incident pressure wave. Therefore, it does not deform in response to the passing pressure wave. The optical fiber D wrapped around the reference mandrel therefore does not stretch or contract in response to the incident wave, and provides a reference path length for the light it carries. Light traversing the stretched sensing fiber B is shifted in phase with respect to light traversing the unstretched reference fiber D. As the pressure wave passes the sensor, the interferometer measures the optical phase shift between the light beams exiting the two fibers B and D. The measured phase difference is proportional to the pressure variation in the reflected acoustic wave.
Hydrophone sensors in common use, such as the one shown in FIG. 1, have several inherent problems and limitations. All these sensors rely on the acoustic pressure acting on a sensing mandrel to induce strain in the fiber. They also rely on an air-filled, compliant cavity between the sensing and reference mandrels to enhance the scale factor. The air filled cavity is formed by sealing the ends of the mandrels to the end caps with epoxy and/or urethane sealant. Deformation of the sensing mandrel as described above significantly strains the rigid epoxy or urethane used to form the seals. In the event that a seal fails, the air cavity becomes flooded with water and the acoustic sensitivity of the hydrophone decreases significantly. Repeated deformation straining of the air cavity seals from repeated use of the sensor in seismic exploration eventually results in fatigue-induced failure of a seal, and of the hydrophone.
Yet another problem with sensors in present use is experienced when the sensors are exposed to high hydrostatic pressures, as when the sensor is placed on the ocean floor. Some current seismic studies use hydrophone sensors at depths up to 3000 meters. The very high hydrostatic pressures encountered at these ocean depths cause their outer mandrels to buckle and the sensors to fail. The probability of failure increases with use of a sensor because the outer, sensing mandrel becomes fatigued by repeated pressure cycling induced deformations.
Consequently, a need exists for a fiber-optic hydrophone sensor having improved performance and life. Specifically, the improved hydrophone sensor should be highly reliable and durable when repeatedly used many times over in severe environments of high hydrostatic pressures.
The present invention addresses the aforementioned need. According to one example embodiment of the invention, there is provided a fiber-optic hydrophone comprising a compliant sensing mandrel and a first optical fiber wound around the compliant sensing mandrel. A rigid reference mandrel is positioned adjacent to the compliant sensing mandrel. A second optical fiber is wound around the rigid reference mandrel. The first and second optical fibers comprise different arms of an interferometer. At least one flexible sealing member seals the compliant sensing mandrel to the hydrophone. A support member is disposed at least partially inside the sensing mandrel. At least a portion of the support member is spaced from the sensing mandrel so as to provide a sealed cavity between the sensing mandrel and the support member.
According to a second example embodiment of the invention, a fiber-optic hydrophone comprises a compliant sensing mandrel and a first optical fiber wound around the compliant sensing mandrel. A rigid reference mandrel surrounds the compliant sensing mandrel. The reference mandrel is spaced from the sensing mandrel so as to provide a sealed cavity therebetween. A second optical fiber is wound around the rigid reference mandrel. The first and second optical fibers comprise different arms of an interferometer. For each mandrel, at least one flexible sealing member seals the mandrel to the hydrophone. A support member is disposed inside the sensing mandrel. The support member is spaced from the sensing mandrel so as to provide a channel therebetween for providing fluid communication therein with the sensing mandrel. Means for providing fluid flow into the channel is also provided.
According to a third example embodiment of the invention, a fiber-optic hydrophone comprises a compliant sensing mandrel and a first optical fiber wound around the compliant sensing mandrel. A rigid reference mandrel is positioned adjacent the compliant sensing mandrel. A second optical fiber is wound around the rigid reference mandrel. The first and second optical fibers comprise different arms of an interferometer. A housing encloses the sensing and reference mandrels and the first and second optical fibers wound thereon. The housing is spaced from the sensing mandrel and first optical fiber so as to provide a sealed cavity therebetween. At least one flexible sealing member seals the housing to at least one of the sensing mandrel and the reference mandrel. A support member is disposed inside the sensing mandrel. The support member is spaced from the sensing mandrel so as to provide a channel therebetween for providing fluid communication therein with the sensing mandrel. Means for providing fluid flow into the channel is also provided.
According to a fourth example embodiment of the invention, a fiber-optic hydrophone comprises a compliant sensing mandrel and a first optical fiber wound around the compliant sensing mandrel. A rigid reference mandrel is positioned inside the sensing mandrel. At least a portion of the reference mandrel is spaced from the sensing mandrel so as to provide a channel therebetween for providing fluid for pressure equalization therein with the sensing mandrel. A second optical fiber is wound around the rigid reference mandrel. The first and second optical fibers comprise different arms of an interferometer. At least one flexible sealing member seals the sensing mandrel to the hydrophone. A tube is in fluid communication with the channel for permitting pressure equalization and frequency roll-off between the exterior of the hydrophone and the channel. The tube responds to D.C. pressure while filtering A.C. pressure of the acoustic signals. The tube responds to hydrostatic pressure while excluding hydrodynamic pressure changes of acoustic signals.
According to a fifth example embodiment of the invention, a fiber-optic hydrophone comprises a compliant sensing mandrel and a first optical fiber wound around the compliant sensing mandrel. A rigid reference mandrel is positioned inside the sensing mandrel. At least a portion of the reference mandrel is spaced from the sensing mandrel so as to provide a sealed cavity between the sensing mandrel and the reference mandrel. A second optical fiber is wound around the rigid reference mandrel. The first and second optical fibers comprise different arms of an interferometer. A pair of O-rings seals the sensing mandrel to the reference mandrel.
According to a sixth example embodiment of the invention, a method for detecting pressure in a marine environment comprises sensing motion of a first body in response to a pressure wave. The first body is in movable contact with a cavity. The cavity is defined, in part, by a first body and a second body. The method further comprises flexibly isolating the cavity from the marine environment at a joint between the first body and the second body.
According to a seventh example embodiment of the invention, a system for detecting pressure in a marine environment comprises means for sensing motion of a first body in response to a pressure wave. The first body is in movable contact with a cavity. The cavity is defined, in part, by a first body and a second body. The system further comprises means for flexibly isolating the cavity from the marine environment at a joint between the first body and the second body.